118 lines
3.9 KiB
Zig
118 lines
3.9 KiB
Zig
const std = @import("index.zig");
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const builtin = @import("builtin");
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const AtomicOrder = builtin.AtomicOrder;
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const AtomicRmwOp = builtin.AtomicRmwOp;
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const assert = std.debug.assert;
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const SpinLock = std.SpinLock;
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const linux = std.os.linux;
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/// Lock may be held only once. If the same thread
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/// tries to acquire the same mutex twice, it deadlocks.
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/// The Linux implementation is based on mutex3 from
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/// https://www.akkadia.org/drepper/futex.pdf
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pub const Mutex = struct {
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/// 0: unlocked
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/// 1: locked, no waiters
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/// 2: locked, one or more waiters
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linux_lock: @typeOf(linux_lock_init),
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/// TODO better implementation than spin lock
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spin_lock: @typeOf(spin_lock_init),
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const linux_lock_init = if (builtin.os == builtin.Os.linux) i32(0) else {};
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const spin_lock_init = if (builtin.os != builtin.Os.linux) SpinLock.init() else {};
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pub const Held = struct {
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mutex: *Mutex,
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pub fn release(self: Held) void {
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if (builtin.os == builtin.Os.linux) {
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const c = @atomicRmw(i32, &self.mutex.linux_lock, AtomicRmwOp.Sub, 1, AtomicOrder.Release);
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if (c != 1) {
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_ = @atomicRmw(i32, &self.mutex.linux_lock, AtomicRmwOp.Xchg, 0, AtomicOrder.Release);
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const rc = linux.futex_wake(&self.mutex.linux_lock, linux.FUTEX_WAKE | linux.FUTEX_PRIVATE_FLAG, 1);
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switch (linux.getErrno(rc)) {
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0 => {},
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linux.EINVAL => unreachable,
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else => unreachable,
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}
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}
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} else {
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SpinLock.Held.release(SpinLock.Held{ .spinlock = &self.mutex.spin_lock });
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}
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}
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};
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pub fn init() Mutex {
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return Mutex{
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.linux_lock = linux_lock_init,
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.spin_lock = spin_lock_init,
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};
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}
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pub fn acquire(self: *Mutex) Held {
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if (builtin.os == builtin.Os.linux) {
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var c = @cmpxchgWeak(i32, &self.linux_lock, 0, 1, AtomicOrder.Acquire, AtomicOrder.Monotonic) orelse
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return Held{ .mutex = self };
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if (c != 2)
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c = @atomicRmw(i32, &self.linux_lock, AtomicRmwOp.Xchg, 2, AtomicOrder.Acquire);
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while (c != 0) {
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const rc = linux.futex_wait(&self.linux_lock, linux.FUTEX_WAIT | linux.FUTEX_PRIVATE_FLAG, 2, null);
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switch (linux.getErrno(rc)) {
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0, linux.EINTR, linux.EAGAIN => {},
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linux.EINVAL => unreachable,
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else => unreachable,
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}
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c = @atomicRmw(i32, &self.linux_lock, AtomicRmwOp.Xchg, 2, AtomicOrder.Acquire);
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}
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} else {
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_ = self.spin_lock.acquire();
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}
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return Held{ .mutex = self };
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}
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};
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const Context = struct {
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mutex: *Mutex,
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data: i128,
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const incr_count = 10000;
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};
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test "std.Mutex" {
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var direct_allocator = std.heap.DirectAllocator.init();
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defer direct_allocator.deinit();
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var plenty_of_memory = try direct_allocator.allocator.alloc(u8, 300 * 1024);
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defer direct_allocator.allocator.free(plenty_of_memory);
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var fixed_buffer_allocator = std.heap.ThreadSafeFixedBufferAllocator.init(plenty_of_memory);
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var a = &fixed_buffer_allocator.allocator;
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var mutex = Mutex.init();
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var context = Context{
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.mutex = &mutex,
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.data = 0,
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};
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const thread_count = 10;
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var threads: [thread_count]*std.os.Thread = undefined;
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for (threads) |*t| {
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t.* = try std.os.spawnThread(&context, worker);
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}
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for (threads) |t|
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t.wait();
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std.debug.assertOrPanic(context.data == thread_count * Context.incr_count);
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}
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fn worker(ctx: *Context) void {
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var i: usize = 0;
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while (i != Context.incr_count) : (i += 1) {
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const held = ctx.mutex.acquire();
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defer held.release();
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ctx.data += 1;
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}
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}
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